It depends. For projects which are never really 'done' (i.e. there's always more polish you can put on), they can help focus things to get them out the door. Ho

It depends. For projects which are never really 'done' (i.e. there's always more polish you can put on), they can help focus things to get them out the door. However for other projects they can be extremely counterproductive, especially if the priority is on having something done by the deadline over having something which is good enough as soon as possible (in which case sufficiently poorly defined deadlines can basically prolong a project indefinitely).